In central utility power plants, once-through steam generators have been extensively used, particularly for large, coal fueled boilers with supercritical steam conditions. These are field erected and built around a vertical radiation section. They typically use membrane tube walls in the radiant section and bare tubes in most of the convective sections. Their cost per unit of heat transfer surface is very high. Combined cycle heat recovery boilers are primarily natural or forced recirculation boilers requiring large steam drums. Virtually all heat recovery boilers, used in combined cycles, employ finned tube heat transfer surfaces arranged in compact convective heat transfer bundles. Most of the large boilers use vertical heat transfer tubes with horizontal gas flow. Large combined cycles typically use thousands of finned tubes from 40 to 60 feet long.
Once-through steam generators, based on innovations recently patented, are now being installed on small combined cycles (5 to 50 MW gas turbines). These small boilers have earned acceptance as the simplest method of implementing the efficient combined cycle into power generation and cogeneration. More than thirty are now in operation. These once-through heat recovery boilers have many circuits of horizontal tubes connected at the feedwater inlet and steam outlet. Water is distributed uniformly into each circuit by an orifice at the inlet of each circuit. As water flows through the horizontal heat transfer tubes it is heated, evaporated and superheated by the turbine exhaust gases flowing vertically up on the finned exterior of the tubes. No water side interconnection is made until the end of each circuit. Each circuit is connected at its end to a steam header that collects steam for distribution to the turbine. Good operational success has been obtained in these installations. The once-through steam generators are the simplest means of recovering waste heat from exhaust gases. They eliminate drums, downcomers, blowdown, boiler water chemical treatment, boiler make-up water, level controls, many valves, and numerous mechanical penetrations of the pressure containment envelope. The once-through boiler can also operate dry, has the highest transient response, reduces make-up water requirements and can be operated unattended.
Due to the numerous advantages they provide, it could be expected that the majority of boilers ordered would be of the once-through type. However, several disadvantages of horizontal tube configurations have limited once-through boilers to small and medium size turbines. More than ninety percent of the operating once-through boilers use an expensive high nickel stainless steel (alloy 800) in their tubing. All once-through steam generators in combined cycle service are horizontal tube configurations to provide drainability to dewater the tubing for freeze and corrosion protection. Several boilers in steam injected gas turbine applications are constructed with only a few rows of vertical tubing. The vertical tube steam injection boilers are in tropical sites and a high nickel stainless steel tubing is used to prevent corrosion. For large gas turbines, boiler tube bundles can weigh more than two million pounds. Thermal expansion during transients requires the tubes to expand several inches. Horizontal tubes, supported by conventional tube sheets, have high sliding frictional forces that place high bending loads on tube sheets and buckling loads on the tubes. To obtain sufficient strength many thick tube sheets are required with resulting high cost. Conventional U-bend end seals to stop blowby losses are further complicated by the greater expansion of long horizontal tubes. This sealing arrangement is expensive and of limited effectiveness where added cross-over tubes are required for triple pressure boilers. Since most large combined cycle boilers have at least triple pressure levels this sealing problem is an important performance consideration.
All the operating once-through boilers have limited capability to accept high supplemental firing temperatures between the gas turbine and the boiler. This limitation is a consequence of the horizontal tube configuration in which the structural support must operate at gas temperatures. As a result, they can only operate to an inlet gas temperature of about 1500.degree. F., even if stainless steel tube sheets are used in their construction. Since the majority of cogeneration applications use supplemental heating, this is a significant limitation.
The above limitations and problems are resolved in practice by using vertical tube natural circulation boilers for large combined cycles. In these natural circulation boilers have self supporting vertical carbon steel tubes cooled by steam or water. The major structure does not operate at gas temperature and the tube weight is directly supported by the tubes. These advantages for large combined cycles have generally made the vertical tube natural circulation boiler the most cost effective approach and most commonly selected.
The novel vertical tube once-through boilers, disclosed herein, are a combination of the above technologies in which the advantages of both types of boiler would be realized. However, before a large vertical tube once-through steam generator becomes practical innovations in the structural, manufacturing, dewatering, materials, operations and maintainability areas must be achieved.
The conventional natural circulation boiler dewaters its vertical tubes by interconnecting every bank of two tube rows with a lower header that has its own drain valve that simultaneously drains two banks of tubes. Lower headers, connecting all of the tube banks, allow a natural circulation boiler to be drained with only about a dozen drain lines. However, vertical tube once-through boilers cannot be economically drained with conventional drain lines and valves. An individual once-through circuit must be kept completely separate from its adjacent circuits. To drain a circuit each lower U-bend tube would need to have its own drain line and valve. Thus, if conventional drains were incorporated, thousands of drain lines and valves would be necessary at the bottom of each U-bend. As a result, all heat recovery once-through boilers in a freezing climate utilize horizontal tubes to be self draining.
Since vertical tube once-through boilers are not practical to drain by gravity, other dewatering methods are necessary. The simplest method is to shut the feedwater flow to completely dry the boiler by evaporating all the water with the turbine exhaust gases prior to scheduled shutdown. Dry operation is an effective method to dewater, and horizontal tube once-through boilers use it to assist in dewatering. However, dry operation to dewater requires the gas turbine to be operational. Thus a back-up system is necessary in case the gas turbine fails.
For large gas turbines, with the exhaust temperatures as high 1200.degree. F., the conventional design of horizontal tube boilers requires numerous thick tube sheet supports and relatively large diameter tubes to prevent tube buckling. The costs are high, particularly for large gas turbines in the 100 to 300 MW size. The high nickel stainless steel tubes, used in conventional once-through boilers, are very expensive relative to the carbon steel tubes used in conventional drum boilers. Both cost and design complexity increase with increasing boiler size. Thus, conventional once-through boilers using high nickel stainless steel tubes, horizontal tubes, and tube sheet construction are limited in application to smaller gas turbines. However, large gas turbines constitute the majority of power generation being installed (based on total MW's). The initial cost of conventional large once-through boilers is the primary problem. To match large combined cycle requirements, and solve the cost problem, an innovative carbon steel once-through boiler with vertical tubes and novel operating methods is necessary and described herein.